Abstract:

Tertiary ethers are formed in reactions between alcohols and alkenes and are used in reformulated gasoline as octane-enhancing agents. By blending ethers into the gasoline pool, less ground-level ozone is formed and combustion of the gasoline is more efficient as a result of the oxygen boost.

The main goal of this research was to study the synthesis of TAME (tert-amyl methyl ether, 2-methoxy-2-methylbutane) and to formulate a kinetic model as precise as possible for process design purposes. The reaction rate was studied as a function of temperature and the reagents feed molar ratio with conventional ion-exchange resin beads and a novel fibrous ion-exchange catalyst. Kinetic modelling favoured the Langmuir-Hinshelwood type model, derived from a dual-site mechanism for the etherification. The influence of the acid capacity of the catalysts on the reaction rate was found to be second order. These results suggest that the etherification reactions occur via a dual-site mechanism.

Comparison of the values of the kinetic parameters obtained with a fibre catalyst and with a bead catalyst indicated that diffusion limitations are associated with the latter. Therefore, mass transfer of the reacting components inside the pores of the cationic ion-exchange resin bead was estimated in terms of the effectiveness factors calculated from experiments with different resin bead sizes. It was concluded that mass transfer has to be taken into account when applying the kinetic model, which was derived for resin beads as the catalyst.

High temperatures and high alcohol concentrations favoured the formation of the dialkyl ethers (dehydration) as a side reaction. When the reaction was maintained in a kinetic regime, it was highly selective for tert-etherification, since the rate of tert-etherification was 140 to 270 times that of dehydration. The experimental results were best described with a model in which one alcohol molecule is adsorbed and the other reacts from the liquid phase. The activation energy was determined to be 102.6 kJ/mol for methanol dehydration to yield DME.

The ranges of validity of the other complete kinetic models proposed for the synthesis of TAME in the literature were evaluated by simulating the experimental conditions and by comparing the adequacy of the models in predicting the experimental composition changes and the composition at reaction equilibrium. Activity-based models were found to predict our experimental results better under a wider range of conditions than concentration-based models.